Yeah, laser scans are cool but the gear to do it costs 50X...
Cover an area that is larger than what you need. The edges get less overlap and thus have lower accuracy.
User Terrain Awareness in your data collection.
Relative measurements should be pretty solid but will certainly have a cumulative error as the distance of the measurement area increases.
Use GCPs to apply linear corrections to the elevation.
120 meters is really high if you want to be getting good detail.
80% overlap should be good but your most recent map is showing overlap of roughly 60% overlap.